With recent rapid progress of information transmitting systems, silver halide photographic light sensitive materials have been increasingly required to have high sensitivity. Such systems are, for example, high speed phototypesetting systems according to which information output from an electronic computer is immediately displayed as letters or figures by a cathode ray tube and press facsimile systems for rapid transmission of news paper originals to a remote place.
Photographic phototypesetting materials include photographic films and papers used in a process for preparing a lithographic printing plate and silver salt diffusion transfer based lithographic printing plates disclosed in e.g. U.S. Pat. No. 4.501.811 and U.S. Pat. No.4.784.933. With the latter materials a lithographic printing plate is immediately obtained without the need of a contact exposure or camera exposure.
Characteristics required for silver halide photographic light sensitive materials for these uses are high sensitivity to so-called high intensity-short time exposure (flash exposure), namely, exposure for 10.sup.-4 second or less with light sources, e.g. laser, cathode ray tube (CRT), helium-neon gas laser and light emission diode (LED), high contrast and high resolving power.
The method of using laser light sources such a helium-neon and argon as a scanning type light source has the defects that large and expensive devices are necessary and efficiency of consumed power is low. On the other hand, a semiconductor laser has the advantages that it is small in size, inexpensive, easily modulatable and long in life. For semiconductor laser, there are used semiconductors of such systems as Ga/As/P; Ga/Al As; Ga/As; In/As and the like and the wavelength of these laser beams is generally longer than 700 nm and largely longer than 750 nm.
Therefore, bright safelight can be used and thus handleability is good. However, light sensitive materials for infrared rays are generally not so high in sensitivity and inferior in shelf stability and various proposals have been made in an attempt to solve these problems. For example, a technique called supersensitization effect has been proposed according to which spectral sensitivity is markedly enhanced by adding a specifically selected organic compound in addition to the spectral sensitizing dye. Reference may be made to, for example, triazine derivatives disclosed in U.S. Pat. Nos. 2,785,058 and 3,695,888, mercapto compounds having electronegative group disclosed i n U.S. Pat. No. 3,457,078. However, many of these supersensitization techniques provide unsatisfactory results such as insufficient sensitization, increase of fog and decrease in shelf stability.